Optical pickup for detecting thickness variation of a recording medium, and/or compensating for spherical aberration caused by thickness variation of a recording medium

ABSTRACT

An optical pickup for a recording medium includes a light beam division and detection unit including receiving portions dividing an incident light beam reflected on the recording medium into a first light beam portion and a second light beam portion around the first light beam portion and converting the first and second light beam portions into first and second detection signals, respectively. A thickness variation detection circuit detects a variation in thickness of the recording medium according to the first and second detection signals and outputs a thickness variation signal indicative thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of Korean Application No.2000-84211, filed Dec. 28, 2000, in the Korean Industrial PropertyOffice, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an optical pickup apparatus, andmore particularly, to an optical pickup for detecting thicknessvariation of a recording medium, and/or for compensating for sphericalaberration caused by the thickness variation of a recording medium.

[0004] 2. Description of the Related Art

[0005] In general, information recording/reproduction density increasesas a size of a light spot focused on a recording medium in an opticalpickup apparatus becomes smaller. The shorter a wavelength (λ) of lightused and the larger a numerical aperture (NA) of an objective lens, thesmaller the size of a light spot, which is expressed by equation (1):

size of light spot α λ/NA  (1)

[0006] To reduce the size of the light spot focused on the recordingmedium in order to obtain a higher recording density, there is a need toconstruct an optical pickup with a short wavelength light source, suchas a blue semiconductor laser, and an objective lens having a larger NA.A format for increasing recording capacity up to 22.5 GB with a 0.85-NAobjective lens, and for reducing the thickness of a recording medium to0.1 mm is desired so as to prevent degradation of performance caused bytilting of the recording medium. Here, the thickness of the recordingmedium is defined as a distance from a light incident surface of therecording medium to an information recording surface.

[0007] As shown in equation (2) below, an spherical aberration W isproportional to a fourth power of the NA of the objective lens and to adeviation of the thickness of the recording medium. For this reason, ifan objective lens with a high NA of about 0.85 is adopted, the recordingmedium must have a uniform-thickness with a deviation less than ±3 μm.However, it is very difficult to manufacture the recording medium withinthe above thickness deviation range. $\begin{matrix}{W_{\lbrack 40\rbrack} = {\frac{n^{2} - 1}{8n^{3}}( {N\quad A} )^{4}\Delta \quad d}} & (2)\end{matrix}$

[0008]FIG. 1 is a graph showing a relation between thickness deviationof the recording medium and wavefront aberration (optical pathdifference (OPD)) caused by a thickness deviation when a 400-nm lightsource and an objective lens having an NA of 0.85 are used. As shown inFIG. 1, the wavefront aberration increases proportionally with thethickness deviation. Thus, when the objective lens having a high NA, forexample, an NA of 0.85, is adopted, there is a need to correct forspherical aberration caused by the thickness deviation of the recordingmedium.

[0009]FIG. 2 shows a conventional optical pickup capable of detectingvariation of the thickness of an optical disc 1, which is disclosed inJapanese Patent Laid-open Publication No. hei 12-57616. Referring toFIG. 2, the conventional optical pickup includes a light source 10emitting a light beam, a polarization beam splitter 11 transmitting orreflecting the light beam from the light source 10 incident on theoptical disc I according to the polarization of the light beam, and aquarter-wave plate 15 changing the polarization of an incident lightbeam. An objective lens 17 focuses the incident light beam to form alight spot on a recording surface 1 a of the optical disc 1. Acylindrical astigmatism lens 21 has an astigmatism affecting the lightbeam passed back through the objective lens 17, the quarter-wave plate15, and the polarization beam splitter 11 after being reflected from therecording surface 1 a of the optical disc 1. A photodetector 25 receivesthe light beam from the astigmatism lens 21. The conventional opticalpickup further includes a collimating lens 13 disposed between thepolarization beam splitter 11 and the quarter-wave plate 15, collimatingan incident diverging light beam from the light source 10 transmitted orreflected by the polarization beam splitter 11. A condensing lens 19 isdisposed between the polarization beam splitter 11 and the astigmatismlens 21. The polarization beam splitter 11, the collimator lens 13, thequarter-wave plate 15, the objective lens 17, the condensing lens 19,and the cylindrical astigmatism lens 21 are coaxially arranged.

[0010] Because the conventional optical pickup has the astigmatism lens21 which causes astigmatism to enable focus error signal detection, theintensity distribution of the light passed through the astigmatism lens21 after being reflected on the recording surface 1 a of the opticaldisc 1 varies according to the thickness t′ of the optical disc 1, asshown in FIGS. 3A through 3E. FIGS. 3A through 3E illustrate anintensity distribution of light passed through the astigmatism lens 21towards the photodetector 25, when the optical disc I adopted has athickness of 0.70 mm, 0.65 mm, 0.60 mm, 0.55 mm, and 0.50 mm,respectively, and the optical pickup of FIG. 2 is designed for a 0.6-mmthick optical disc.

[0011] Referring to FIG. 3C, when the optical disc 1 has a thickness of0.60 mm, which is a level of reference with respect to the otherthickness levels (hereinafter, referred to as the reference thickness),the intensity distribution of the reflected light beam entering thephotodetector 25 is circular due to lack of spherical aberration, and issymmetrical around a center point. When the thickness of the opticaldisc 1 deviates from the reference thickness of 0.60 mm, sphericalaberration occurs as a result of the thickness deviation, and theintensity distribution of the reflected light beam passed through theastigmatism lens 21 and received by the photodetector 25 is asymmetricalabout the center point, as illustrated in FIGS. 3A, 3B, 3D, and 3E.

[0012] The photodetector 25 detects a variation in thickness of theoptical disc 1 from a variation of intensity distribution of thereceived light. To this end, as shown in FIG. 4, the photodetector 25 ofFIG. 2 includes first through fourth inner sections A1, B1, C1, and D1,and first through fourth outer sections A2, B2, C2, and D2 surroundingthe first through fourth inner sections A1, B1, C1, and D1.

[0013] In a conventional optical pickup having the configurationdescribed above, a thickness variation signal for the optical disc 1 isdetected by subtracting a sum of detection signals a2 and c2 of thefirst and third outer sections A2 and C2 in one diagonal direction ofthe photodetector 25, and the detection signals b1 and d1 of the secondand fourth inner sections B1 and D1, respectively, in the other diagonaldirection, from a sum of detection signals a1 and c1 of the first andthird inner sections A1 and C1, respectively, in the one diagonaldirection, and detection signals b2 and d2 of the second and fourthouter sections B2 and D2, respectively, in the other diagonal direction.In other words, a thickness variation signal St′ for the optical disc 1can be detected from the detection signals a1, b1, c1, and d1 of thefirst through fourth inner sections A1, B1, C1, and D1, respectively, ofthe photodetector 25, and the detection signals a2, b2, c2, and d2 ofthe first through fourth outer sections A2, B2, C2 and D2, respectively,by using the following equation:

St=(a 1+c 1+b 2+d 2)−(a 2+c 2+b 1+d 1)  (3)

[0014] However, this mechanism of detecting variation of the thicknessof the optical disc can be applied to only optical pickups adopting theastigmatism lens. In other words, if an optical pickup does not includethe astigmatism lens, a thickness variation of an optical disc used inthe optical pickup cannot be detected.

SUMMARY OF THE INVENTION

[0015] Various objects and advantages of the invention will be set forthin part in the description that follows and, in part, will be obviousfrom the description, or may be learned by practice of the invention.

[0016] To solve the above problem, it is an object of the presentinvention to provide an optical pickup detecting variation of thethickness of a recording medium, and/or compensating for sphericalaberration caused by the thickness variation of a recording medium,without including an astigmatism lens to cause astigmatism at the lightreceiving side.

[0017] According to an object of the present invention, there isprovided an optical pickup including: a light source generating andemitting a light beam; an objective lens focusing the light beam fromthe light source to form a light spot incident on the recording medium;an optical path changer disposed on an optical path between the lightsource and the objective lens, altering a traveling path of the incidentlight beam; a light beam division and detection unit dividing theincident light beam passed through the objective lens and the opticalpath changer into a first light beam portion and a second light beamportion around the first light beam portion, and detecting first andsecond detection signals from the first and second light beam portions;and a thickness variation detection circuit detecting a variation inthickness of the recording medium by subtracting the second detectionsignal from the first detection signal and outputting a thicknessvariation signal indicative thereof.

[0018] The light beam division and detection unit may include aphotodetector including first and second light receiving portionsdividing the incident light beam into the first light beam portion andthe second light beam portion around the first light beam portion andphotoelectrically converting the first and second light beam portionsinto the first and second detection signals, respectively.

[0019] Preferably, the light beam division and detection unit includes:a light beam splitter including a first section and a second section,dividing the incident light beam into the first light beam portion andthe second light beam portion around the first light beam portion; andfirst and second photodetectors receiving the first and second lightbeam portions from the light beam splitter, and photoelectricallyconverting the first and second light beam portions into the first andsecond detection signals, respectively.

[0020] In another embodiment, the present invention provides for anoptical pickup including: a light source generating and emitting a lightbeam; an objective lens focusing an incident light beam from the lightsource to form a light spot on the recording medium; an optical pathchanger disposed on an optical path between the light source and theobjective lens, altering a traveling path of the incident light beam; alight beam division and detection unit dividing the incident light beampassed through the objective lens and the optical path changer into afirst light beam portion and second and third light beam portions aroundthe first light beam portion, and detecting first, second, and thirddetection signals from the first, second and third light beam portions,respectively; a thickness variation detection circuit detecting avariation in thickness of the recording medium by subtracting a sum ofthe second and third detection signals from the first detection signaland outputting a thickness variation signal indicative thereof.

[0021] The light beam division and detection unit includes aphotodetector including first, second and third light receiving portionsdividing the incident light beam into the first, second and third lightbeam portions, receiving the first, second and third light beamportions, and separately and photoelectrically converting the first,second and third light beam portions, respectively.

[0022] The light beam division and detection unit includes: a light beamsplitter including first, second, and third sections dividing theincident light beam into the first light beam portion and the second andthird light beam portions around the first light beam portion; a firstphotodetector receiving and photoelectrically converting the first lightbeam portion into the first detection signal; a second photodetectorreceiving and photoelectrically converting the second light beam portioninto the second detection signal; and a third photodetector receivingand photoelectrically converting the third light beam portion into thesecond detection signal.

[0023] The optical pickup according to the present invention may furtherinclude a spherical aberration compensation element on the optical pathbetween the optical path changer and the objective lens, drivenaccording to the thickness variation signal from the thickness variationdetection circuit to compensate for spherical aberration caused by thethickness variation of the recording medium.

[0024] The optical pickup according to the present invention may furtherinclude: a collimating lens on the optical path between the light sourceand the optical path changer, collimating a diverging light beam fromthe light source; and an actuator actuating the collimating lensaccording to the thickness variation signal detected by the thicknessvariation detection circuit, compensating for spherical aberrationcaused by the thickness variation of the recording medium.

[0025] It is another object of the present invention to provide for anoptical pickup for a recording medium, including: a light beam divisionand detection unit including receiving portions dividing an incidentlight beam reflected on the recording medium into a first light beamportion and a second light beam portion around the first light beamportion and converting the first and second light beam portions intofirst and second detection signals, respectively; and a thicknessvariation detection circuit detecting a variation in thickness of therecording medium according to the first and second detection signals andoutputting a thickness variation signal indicative thereof.

[0026] These together with other objects and advantages, which will besubsequently apparent, reside in the details of construction andoperation as more fully hereinafter described and claimed, referencebeing had to the accompanying drawings forming a part thereof, whereinlike numerals refer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The above object and advantages of the present invention willbecome more apparent by describing in detail preferred embodimentsthereof with reference to the attached drawings in which:

[0028]FIG. 1 is a graph showing the relation between thickness deviationof a recording medium and wavefront aberration (optical path difference(OPD)) caused by the thickness deviation;

[0029]FIG. 2 illustrates a conventional optical pickup detectingthickness variation of an optical disc;

[0030]FIG. 3A illustrates a distribution of light entering aphotodetector of the optical pickup of FIG. 2, which is designed for0.60-mm thick optical discs and the optical disc has a thickness of 0.70mm;

[0031]FIG. 3B illustrates a distribution of light entering thephotodetector of the optical pickup of FIG. 2, which is designed for0.60-mm thick optical discs and the optical disc has a thickness of 0.65mm;

[0032]FIG. 3C illustrates a distribution of light entering thephotodetector of the optical pickup of FIG. 2, which is designed for0.60-mm thick optical discs and the optical disc has a thickness of 0.60mm;

[0033]FIG. 3D illustrates a distribution of light entering thephotodetector of the optical pickup of FIG. 2, which is designed for0.60-mm thick optical discs and the optical disc has a thickness of 0.55mm;

[0034]FIG. 3E illustrates a distribution of light entering thephotodetector of the optical pickup of FIG. 2, which is designed for0.60-mm thick optical discs and the optical disc has a thickness of 0.50mm;

[0035]FIG. 4 is a plan view illustrating a configuration of thephotodetector shown in FIG. 2;

[0036]FIG. 5 shows an exemplary embodiment of an optical pickup inaccordance with the present invention;

[0037]FIG. 6A illustrates an intensity distribution of a light beampassed back through an objective lens and an optical path changer of theoptical pickup of FIG. 5 after being reflected from the recordingmedium, which is 10 μm thinner than a reference thickness for which theoptical pickup is designed;

[0038]FIG. 6B illustrates an intensity distribution of the light beampassed back through the objective lens and the optical path changer ofthe optical pickup of FIG. 5 after being reflected from the recordingmedium having a reference thickness of 0.1 mm;

[0039]FIG. 6C illustrates an intensity distribution of the light beampassed back through the objective lens and the optical path changer ofthe optical pickup of FIG. 5 after being reflected from the recordingmedium, which is 10 μm thicker than the reference thickness for whichthe optical pickup is designed;

[0040]FIG. 7A illustrates a phase distribution of the light beam of FIG.6A;

[0041]FIG. 7B illustrates a phase distribution of the light beam of FIG.6B;

[0042]FIG. 7C illustrates a phase distribution of the light beam of FIG.6C;

[0043]FIG. 8 illustrates exemplary embodiment of a photodetector of FIG.5 and a thickness variation detection circuit;

[0044]FIG. 9 illustrates an alternative embodiment of the thicknessvariation detection circuit of FIG. 8;

[0045]FIG. 10 illustrates alternative embodiment of the photodetectorand the thickness variation detection circuit of FIG. 5;

[0046]FIG. 11 is a graph of a thickness variation signal and a sum offirst and second detection signals of first and second light receivingportions of the photodetector when the photodetector of the opticalpickup, according to the present invention, has the embodiment of FIG.8;

[0047]FIG. 12 is a graph of a thickness variation signal for therecording medium and a sum of the first, second, and third detectionsignals of first, second and third light receiving portions of thephotodetector when the photodetector of the optical pickup, according tothe present invention, has the embodiment of FIG. 10;

[0048]FIG. 13 illustrates an alternative embodiment of the opticalpickup according to the present invention;

[0049]FIG. 14 illustrates an alternative embodiment of the opticalpickup according to the present invention;

[0050]FIG. 15 is a plan view showing an embodiment of a light beamsplitter of FIG. 14;

[0051]FIG. 16 illustrates an alternative embodiment of the opticalpickup according to the present invention; and

[0052]FIG. 17 is a plan view showing an embodiment of the light beamsplitter of FIG. 16.

DETAILED DESCRIPTION OF THE INVENTION

[0053] An exemplary embodiment of an optical pickup according to thepresent invention is illustrated in FIG. 5. The optical pickup includesa light source 51 generating and emitting a light beam, and an objectivelens 57 focusing an incident light beam LB from the light source 51 toform a light spot on an information recording surface 50 a of arecording medium 50. An optical path changer is disposed on the opticalpath between the light source 51 and the objective lens 57, for alteringthe traveling path of the incident light beam LB. A photodetector 65 isprovided to divide and detect the light beam passed back through theobjective lens 57 and the optical path changer after being reflectedfrom the recording medium 50, and a thickness variation detectioncircuit 70 is provided to detect variation of the thickness of therecording medium 50 from a plurality of detection signals output fromthe photodetector 65. Here, the thickness t of the recording medium 50is defined as a distance between a light incident surface 50 b of therecording medium 50 and the information recording surface 50 a.Thickness variation refers to both thickness deviation according toposition on one recording medium and a difference in thickness betweendifferent optical discs.

[0054] The light source 51 may be a semiconductor laser, such as an edgeemitting laser or a vertical cavity surface emitting laser (VCSEL). Asthe optical path changer, a beam splitter 55 for transmitting andreflecting an incident light beam LB by a predetermined ratio may beadopted. Alternatively, the optical path changer may include both, apolarization beam splitter (not shown) for selectively transmitting orreflecting an incident light beam LB according to a polarization of theincident light beam LB, and a quarter-wave plate (not shown) disposed onthe optical path between the polarization beam splitter and theobjective lens 57, for changing the phase of an incident light beam LB.

[0055] In the optical pickup of FIG. 5 for recording and reproductionwith a next generation digital versatile disc (DVD), so-called“high-definition (HD)-DVD”family recording medium, a blue-lightsemiconductor laser may be adopted as the light source 51 to emit alight beam having a wavelength of about 400-420 nm, for instance, awavelength of about 405 nm, and a lens having a numerical aperture (NA)of 0.7 or more, such as, an NA of 0.85, may be adopted as the objectivelens 57.

[0056] The optical pickup according to the present invention may furtherinclude a collimating lens 53 on the optical path between the lightsource 51 and the beam splitter 55, for collimating a diverging lightbeam emitted from the light source 51. The optical pickup furtherincludes a sensing lens 59 on the optical path between the beam splitter55 and the photodetector 65, for condensing an incident light beam LB.The distance between the sensing lens 59 and the photodetector 65 isdetermined such that the light spot received by the photodetector 65 hasan appropriate size, for example, a diameter of about 100 μm.

[0057] The photodetector 65 as a light beam division and detection unit,is constructed such that the photodetector 65 divides and detects thelight beam passed back through the objective lens 57 and the opticalpath changer after being reflected on the recording medium 50, takinginto account a variation in the intensity distribution of the light beamaccording to thickness variation of the recording medium 50.

[0058] For illustrative purposes, the objective lens 57 has an NA of0.85, the optical pickup is designed for the recording medium 50 havinga thickness of 0.1 mm, and the light source 51 emits a 400-nm lightbeam. In this case, FIGS. 6A through 6C and FIGS. 7A through 7Cillustrate intensity distribution and phase distribution of the lightbeam passed back through the objective lens 57 and the optical pathchanger after being reflected from the recording medium 50, with respectto the variation in the thickness of the recording medium 50.

[0059] In particular, FIG. 6A illustrates the intensity distribution ofthe light passed through the optical path changer after being reflectedon the recording medium 50 which is 10 μm thinner than the 0.1 mmthickness for which the optical pickup is designed (hereinafter referredto as the reference thickness). FIG. 7A illustrates the phasedistribution of the light beam of FIG. 6A. FIG. 6B illustrates theintensity distribution of the light beam for the recording medium 50having a thickness equal to the reference thickness of 0.1 mm. FIG. 7Bshows the phase distribution of the light beam of FIG. 6B. FIG. 6Cillustrates the intensity distribution of the light beam for therecording medium 50, which is 10 μm thicker than the referencethickness. FIG. 7C illustrates the phase distribution of the light beamof FIG. 6C.

[0060] Referring to FIGS. 6A and 7A, when a portion of the recordingmedium 50, onto which the light beam is focused, is thinner than thereference thickness, the intensity distribution of the light beam isweaker at a central axis, and increases with increased distance from thecentral axis. Also, the phase distribution of the light beam appearslike twin peaks, that is, symmetrical with respect to the central axis.Referring to FIGS. 6B and 7B, when a portion of the recording medium 50onto which the light beam is focused is equal to the referencethickness, the intensity distribution of the light beam is uniform andthe phase distribution is uniform. Referring to FIGS. 6C and 7C, when aportion of the recording medium 50 onto which the light beam is focusedis thicker than the reference thickness, the intensity distribution andthe phase distribution of the light beam are inverted with respect tothose of FIGS. 6A and 7A.

[0061] As illustrated in FIGS. 6A through 6C and FIGS. 7A through 7C,the intensity distribution and the phase distribution of the light beamaccording to the thickness variations of the recording medium 50 varysymmetrically around the central axis and are opposite with respect toan inverse in thickness variations. Furthermore, variations in theintensity distribution and phase spectrum of the light beam caused by anincrease in the thickness of the recording medium above the referencethickness are opposite to variation in the intensity distribution andthe phase spectrum of the light beam caused by a decrease in thethickness of the recording medium below the reference thickness.

[0062] For this reason, the photodetector 65 may be constructed suchthat the photodetector 65 separately divides the incident light beam LBinto a first light portion corresponding to the optical axis and asecond light portion around the periphery of the optical axis. Forexample, as shown in FIG. 8, the photodetector 65 may include first andsecond light receiving portions A and B, respectively, for dividing theincident light beam LB into a first light beam portion corresponding tothe central optical axis, and a second light beam portion around thefirst light beam portion, and for photoelectrically converting the firstand second light beam portions into first and second detection signals aand b, respectively. In this case, the first light receiving portion Aof the photodetector 65 may have a circular or rectangular form to allowdivision of the incident light beam LB into the first light beamportion, corresponding to the central optical axis, and the second lightbeam portion around the first light beam portion, and to allow separatedetection of the two portions.

[0063] When the photodetector 65 includes the first and second lightreceiving portions A and B, as shown in FIG. 8, the thickness variationdetection circuit 70 is constructed with a subtractor 71 for subtractingthe second detection signal b corresponding to the second light beamportion of the second light receiving portion B from the first detectionsignal a corresponding to the first light beam portion of the firstlight receiving portion A and outputting a result of the subtraction asa thickness variation signal St to the recording medium 50. In thiscase, as shown in FIG. 9, the thickness variation detection circuit 70may further include a gain controller 73 for amplifying at least one ofthe first and second detection signals a and b by a predetermined gainfactor k prior to the subtraction by the subtractor 71, such that anoffset of the thickness variation signal St can be adjusted.

[0064] Alternatively, as shown in FIG. 10, the photodetector 65 mayinclude first, second, and third light receiving portions D, E, and F,respectively, for dividing the incident light beam LB into a first lightbeam portion aligned with an optical axis, and second and third lightbeam portions around the first light beam portion, and forphotoelectrically converting the first, second, and third light beamportions into first, second, and third detection signals d, e, and f,respectively. The first, second, and third light receiving portions D,E, and F may be arranged in a direction corresponding to either atangential or radial direction of the recording medium 50.

[0065] When the photodetector 65 is constructed as illustrated in FIG.10, the thickness variation detection circuit 70 detects the variationin the thickness of the recording medium 50 by subtracting the sum ofthe second and third detection signals e and f, of the second and thirdlight receiving portions E and F, from the first detection signal d ofthe first light receiving portion D. As illustrated in FIG. 9, thethickness variation detection circuit 70 may be constructed such thatthe thickness variation detection circuit 70 may amplify at least one ofthe first, second and third detection signals d, e, and f, respectively,by a predetermined gain factor k, and process the first, second, andthird detection signals d, e, and f, respectively, so that the offset ofthe thickness variation signal St may be adjusted.

[0066] For the photodetector 65 illustrated in FIGS. 8 and 10, a size ofthe first light receiving portion A of FIG. 8 and a size of the firstlight receiving portion D of FIG. 10 are determined such that the firstlight receiving portion A and the first light receiving portion Dreceive 10-90% of the entire incident light beam LB.

[0067] Turning back to FIG. 5, the optical pickup according to thepresent invention may further include a spherical aberrationcompensation element 75 on the optical path between the optical pathchanger and the objective lens 57, which is driven according to thethickness variation signal St produced by the thickness variationdetection circuit 70, thereby compensating for spherical aberrationcaused by thickness variation of the recording medium 50.

[0068] The spherical aberration compensation element 75 may be a liquidcrystal plate manufactured by injecting liquid crystals between twosheets of transparent substrates having electrode patterns. Due to theanisotropic property of the liquid crystal with respect to a refractiveindex, the phase of the light beam passing through the liquid crystalplate changes. In particular, the liquid crystal plate is drivenaccording to the thickness variation signal St such that a shape of awavefront of the light beam passing the liquid crystal plate is changedinto an inverse shape of spherical aberration caused by the thicknessvariation of the recording medium 50, thereby compensating for thespherical aberration caused by the thickness variation of the recordingmedium 50. In this case, a driving circuit for driving the sphericalaberration compensation element 75 may be included in or separate fromthe thickness variation detection circuit 70.

[0069]FIG. 11 is a graph of the thickness variation signal St and a sumSsum of the first and second detection signals a and b of the first andsecond light receiving portions A and B of the photodetector 65 versusthe thickness variation of the recording medium 50 when thephotodetector 65 of the optical pickup of the present invention has theembodiment of FIG. 8. FIG. 12 is a graph of the thickness variationsignal St and the sum Ssum of the first, second, and third detectionsignals d, e, and f of the first, second, and third light receivingportions D, E, and F of the photodetector 65 versus the thicknessvariation of the recording medium 50, when the photodetector 65 of theoptical pickup of the present invention has the embodiment of FIG. 10.As shown in FIGS. 11 and 12, the variation of the thickness variationsignal St detected by the thickness variation detection circuit 70 withrespect to the thickness variation of the recording medium 50 isrelatively larger than the variation of the sum Ssum of the detectionsignals detected by the photodetector 65.

[0070] As described with reference to FIGS. 11 and 12, the variation inthe thickness of the recording medium 50 can be detected by the opticalpickup of the present invention. Thus, the spherical aberration causedby the thickness variation of the recording medium 50 can be correctedby driving the spherical aberration compensation element 75 according tothe thickness variation signal St.

[0071] Referring to FIG. 13, for the purpose of compensating forspherical aberration caused by the thickness variation of the recordingmedium 50, the optical pickup according to the present invention mayinclude an actuator 80 actuating the collimating lens 53 along theoptical axis according to the thickness variation signal St produced bythe thickness variation detection circuit 70 instead of the sphericalaberration correction element 75 of FIG. 5.

[0072]FIG. 14 illustrates an alternative embodiment of the opticalpickup according to the present invention. In the present embodiment,rather than the photodetector 65 having the embodiment shown in FIG. 8,a light beam splitter 160, and first and second photodetectors 165 a and165 b are used as a light beam division and detection unit. In FIG. 14,the same elements as in FIG. 5 are denoted by the same referencenumerals, and descriptions thereof will not be provided here.

[0073] The light beam splitter 160 includes first and second sections A′and B′ as shown in FIG. 15 for dividing the incident light beam LB intoa first light beam portion on the optical axis, and a second light beamportion around the first light beam portion. The first section Adirectly transmits, for example, the first light beam portion of theincident light beam LB, or diffracts the first light beam portion of theincident light beam LB into a 0th-order beam, so that the transmitted ordiffracted light beam is received by the first photodetector 165 a. Thesecond section B′ diffracts, for example, the second light beam portionof the incident light beam LB so that a +1^(st)-order or −1^(st)-orderbeam is received by the second photodetector 165 b. The light beamsplitter 160 may be a hologram optical element (HOE), which has in thefirst section A′ a through hole, a direct transmit portion, or ahologram pattern for diffracting the incident light beam LB andtransmitting a resulting 0^(th)-order light beam, and has in the secondsection B′ a hologram pattern for diffracting the incident light beam LBand transmitting a resulting +1^(st)-order or −1^(st)-order light beam.

[0074] In the optical pickup according to the embodiment of the presentinvention shown in FIG. 14, the principle of detecting the thicknessvariation signal St for the recording medium 50 from the first andsecond detection signals a and b of the first and second photodetectors165 a and 165 b, and correcting the spherical aberration caused by thethickness variation of the recording medium 50 by driving the sphericalaberration compensation element 75 according to the thickness variationsignal St, is the same as in the previous embodiments. Alternatively,the optical pickup of FIG. 14 may include the actuator 80 for actuatingthe collimating lens 53 along the optical axis, as shown in FIG. 13,thereby compensating for spherical aberration caused by thicknessvariation of the recording medium 50.

[0075]FIG. 16 illustrates an alternative embodiment of the opticalpickup according to the present invention. In the present embodiment,instead of the photodetector 65 having the divided configuration shownin FIG. 10, a light beam splitter 260, and first, second and thirdphotodetectors 265 d, 265 e, and 265 f are used as a light beam divisionand detection units.

[0076] In FIG. 16, the same elements as in FIG. 5 are denoted by thesame reference numerals, and descriptions thereof will not be providedhere.

[0077] In the embodiment shown in FIG. 16, the light beam splitter 260includes first, second and third sections D′, E′ and F′, as shown inFIG. 17, for dividing the incident light beam LB into a first light beamportion on the optical axis, and second and third light beam portionsaround the first light beam portion, based on a principle similar to thelight beam splitter 160 of FIG. 15. The first section D′directlytransmits, for example, the first light beam portion of the incidentlight beam LB, or diffracts the first light beam portion of the incidentlight beam LB and transmits the resulting 0^(th)-order beam, so that thetransmitted or diffracted light beam is received by the firstphotodetector 265 d. The second section E′ diffracts, for example, thesecond light beam portion of the incident light beam LB so that the+1^(st)-order or −1^(st)-order diffracted light beam is received by thesecond photodetector 265 e. The third section F′ diffracts, for example,the third light beam portion of the incident light beam LB so that the−1^(st)-order or +1^(st)-order diffracted light beam is received by thethird photodetector 265 f. The light beam splitter 160 may be an HOE,which has in the first section D′a through hole, a direct transmitportion, or a hologram pattern for diffracting the incident light beamLB, and has in both, the second and third sections E′ and F′, a hologrampattern for diffracting the incident light beam LB.

[0078] In the optical pickup according to the embodiment of the presentinvention shown in FIG. 16, the principle of detecting the thicknessvariation signal St for the recording medium 50 from the first, second,and third detection signals d, e, and f of the first, second, and thirdphotodetectors 265 d, 265 e, and 265 f, and compensating for thespherical aberration caused by thickness variation of the recordingmedium 50 is the same as the principle described in the previousembodiments. In particular, the principle of detecting the thicknessvariation signal St for the recording medium 50 used by the embodimentof FIG. 16 includes driving the spherical aberration compensationelement 75 or the collimating lens 53 according to the detectedthickness variation signal St.

[0079] As described above, in the optical pickups according to thepresent invention, a single photodetector having a divided configurationor a light beam splitter and a plurality of photodetectors is used as alight beam division and detection unit. The light beam passed through anobjective lens and an optical path changer after having been reflectedfrom a recording medium is divided and detected by the light beamdivision and detection unit, taking into account a variation inintensity distribution of the light caused by thickness variation of therecording medium. A thickness variation signal is detected by processingthe detection signals from the photodetector. Thus, the variation inthickness of the recording medium can be detected with the opticalsystem without an astigmatism lens installed to cause astigmatism at thelight receiving side of the optical pickup. Spherical aberration causedby the thickness variation of the recording medium can be corrected bydriving a spherical aberration compensation element or a collimatinglens along the optical axis according to the detected thicknessvariation signal.

[0080] While this invention has been particularly shown and describedwith reference to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made thereto without departing from the spirit and scope of theinvention as defined by the appended claims.

What is claimed is:
 1. An optical pickup for a recording medium,comprising: a light source generating and emitting a light beam; anobjective lens focusing the light beam from the light source to form alight spot on the recording medium; an optical path changer disposed onan optical path between the light source and the objective lens,altering a traveling path of the light beam incident on the recordingmedium; a light beam division and detection unit dividing the incidentlight beam passed through the objective lens and the optical pathchanger after being reflected from the recording medium into a firstlight beam portion and a second light beam portion around the firstlight beam portion, and detecting first and second detection signalsfrom the first and second light beam portions; and a thickness variationdetection circuit detecting a variation in thickness of the recordingmedium by subtracting the second detection signal from the firstdetection signal and outputting a thickness variation signal indicativethereof.
 2. The optical pickup as recited in claim 1, wherein the lightbeam division and detection unit comprises: a photodetector comprisingfirst and second light receiving portions dividing the incident lightbeam into the first light beam portion and the second light beam portionaround the first light beam portion and photoelectrically converting thefirst and second light beam portions into the first and second detectionsignals, respectively.
 3. The optical pickup as recited in claim 1,wherein the light beam division and detection unit comprises: a lightbeam splitter comprising a first section and a second section, dividingthe incident light beam into the first light beam portion and the secondlight beam portion around the first light beam portion; and first andsecond photodetectors receiving the first and second light beam portionsfrom the light beam splitter, and photoelectrically converting the firstand second light beam portions into the first and second detectionsignals, respectively.
 4. The optical pickup as recited in claim 3,wherein the first section transmits the first light beam portion of theincident light beam or diffracts the first light beam into a0^(th)-order beam and the first photodetector receives the transmittedor diffracted first light beam.
 5. The optical pickup as recited inclaim 3, wherein the second section diffracts the second light beamportion of the incident light beam into a +1^(st)-order or a−1^(st)-order beam and the second photodetector receives the diffractedsecond light beam.
 6. The optical pickup as recited in claim 3, whereinthe light beam splitter comprises in the first section a through hole, adirect transmit portion, or a hologram pattern to diffract the incidentlight beam and to transmit a resulting 0^(th)-order light beam to thefirst photodetector, and comprises in the second section a hologrampattern to diffract the incident light beam and to transmit a resulting+1^(st)-order or −1^(st)-order light beam to the second photodetector.7. The optical pickup as recited in claim 1, wherein the light beamdivision and detection unit divides and detects the light beam as acircular or rectangular first light beam portion and as a circular orrectangular second light beam portion surrounding the first light beamportion.
 8. The optical pickup as recited in claim 7, wherein the firstlight beam portion corresponds to 10-90% of the incident light beam. 9.The optical pickup as recited in claim 1, wherein the thicknessvariation detection circuit amplifies at least one of the first andsecond detection signals by a predetermined gain factor, and processesthe at least one of the first and second detection signals to detect thethickness variation of the recording medium.
 10. The optical pickup asrecited in claim 1, further comprising: a spherical aberrationcompensation element on the optical path between the optical pathchanger and the objective lens, driven according to the thicknessvariation signal from the thickness variation detection circuit tocompensate for spherical aberration caused by the thickness variation ofthe recording medium.
 11. The optical pickup as recited in claim 10,further comprising: a collimating lens on the optical path between thelight source and the optical path changer, collimating the light beam,which is diverging, from the light source; and an actuator actuating thecollimating lens according to the thickness variation signal detected bythe thickness variation detection circuit compensating for the sphericalaberration caused by the thickness variation of the recording medium.12. The optical pickup as recited in claim 11, wherein the sphericalaberration compensation element comprises: a liquid crystal plate; and adriving circuit driving the spherical aberration compensation element tocompensate for spherical aberration caused by the thickness variation ofthe recording medium by changing a shape of a wavefront of the lightbeam passing the liquid crystal plate by changing the wavefront of thelight beam into an inverse shape.
 13. The optical pickup as recited inclaim 1, wherein the thickness of the recording medium is a distancebetween a light incident surface of the recording medium and aninformation recording surface of the recording medium.
 14. The opticalpickup as recited in claim 2, wherein the optical path changercomprises: a beam splitter transmitting the light beam from the lightsource to the recording medium through the objective lens and reflectingthe incident light beam through the objective lens to the photodetectorby a predetermined ratio.
 15. The optical pickup as recited in claim 1,further comprising: a collimating lens on the optical path between thelight source and the optical path changer collimating the light beam,which is diverging, from the light source; and a sensing lens on theoptical path between the optical path changer and the light beamdivision and detection unit condensing the incident light beam:
 16. Theoptical pickup as recited in claim 1, wherein the optical path changercomprises: a polarization beam splitter selectively transmitting thelight beam from the light source to the recording medium and reflectingthe incident light beam to the photodetector according to a polarizationof the incident light beam; and a quarter-wave plate disposed on theoptical path between the polarization beam splitter and the objectivelens changing a phase of the incident light beam.
 17. The optical pickupas recited in claim 1, wherein the thickness variation detection circuitfurther comprises: a gain controller amplifying at least one of thefirst and second detection signals by a predetermined gain factor kprior to subtracting the second detection signal from the firstdetection signal, adjusting an offset of the thickness variation signal.18. The optical pickup as recited in claim 1, further comprising: ablue-light semiconductor laser emitting the light beam having awavelength between 400 nm and 420 nm, wherein the objective lenscomprises a numerical aperture of at least 0.7.
 19. The optical pickupas recited in claim 1, wherein the light beam division and detectionunit comprises: a photodetector comprising first, second, and thirdlight receiving portions to divide the incident light beam into thefirst light beam portion, aligned with an optical axis, and the secondand third light beam portions around the first light beam portion and tophotoelectrically convert the first, second, and third light beamportions into the first, second, and third detection signals.
 20. Theoptical pickup as recited in claim 19, wherein the first, second, andthird light receiving portions are arranged in a direction correspondingto either a tangential or radial direction of the recording medium. 21.The optical pickup as recited in claim 2, wherein a size of the lightreceiving portion is determined where the first light receiving portionreceives 10-90% of the entire incident light beam.
 22. An optical pickupfor a recording medium, comprising: a light source generating andemitting a light beam; an objective lens focusing an incident light beamfrom the light source to form a light spot on the recording medium; anoptical path changer disposed on an optical path between the lightsource and the objective lens, altering a traveling path of the lightbeam incident on the recording medium; a light beam division anddetection unit dividing the incident light beam passed through theobjective lens and the optical path changer after being reflected fromthe recording medium into a first light beam portion and second andthird light beam portions around the first light beam portion, anddetecting first, second, and third detection signals from the first,second and third light beam portions, respectively; a thicknessvariation detection circuit detecting a variation in thickness of therecording medium by subtracting a sum of the second and third detectionsignals from the first detection signal and outputting a thicknessvariation signal indicative thereof.
 23. The optical pickup as recitedin claim 22, wherein the light beam division and detection unitcomprises: a photodetector comprising first, second, and third lightreceiving portions dividing the incident light beam into the first,second and third light beam portions, receiving the first, second andthird light beam portions, and separately and photoelectricallyconverting the first, second and third light beam portions,respectively.
 24. The optical pickup as recited in claim 22, wherein thethickness variation detection circuit amplifies at least one of thefirst, second, and third detection signals by a predetermined gainfactor, and processes the first, second and third detection signals todetect the thickness variation of the recording medium.
 25. The opticalpickup as recited in claim 22, wherein the first light beam portioncorresponds to 10-90% of the incident light beam.
 26. The optical pickupas recited in claim 22, wherein the light beam division and detectionunit comprises: a photodetector comprising first, second, and thirdlight receiving portions to divide the incident light beam into thefirst light beam portion, aligned with an optical axis, and the secondand third light beam portions around the first light beam portion and tophotoelectrically convert the first, second, and third light beamportions into the first, second, and third detection signals.
 27. Theoptical pickup as recited in claim 23, wherein the first, second, andthird light receiving portions are arranged in a direction correspondingto either a tangential or radial direction of the recording medium. 28.The optical pickup as recited in claim 22, wherein the thicknessvariation detection circuit amplifies one of the first, second, andthird detection signals by a predetermined gain factor, and processesthe one of the first, second, and third detection signals to detect thethickness variation of the recording medium.
 29. The optical pickup asrecited in claim 22, wherein the light beam division and detection unitcomprises: a light beam splitter comprising first, second, and thirdsections dividing the incident light beam into the first light beamportion and the second and third light beam portions around the firstlight beam portion; a first photodetector receiving andphotoelectrically converting the first light beam portion into the firstdetection signal; a second photodetector receiving and photoelectricallyconverting the second light beam portion into the second detectionsignal; and a third photodetector receiving and photoelectricallyconverting the third light beam portion into the second detectionsignal.
 30. The optical pickup as recited in claim 29, wherein the firstsection directly transmits the first light beam portion of the incidentlight beam or diffracts the first light beam into 0^(th)-order beam andthe first photodetector receives the transmitted or diffracted firstlight beam.
 31. The optical pickup as recited in claim 29, wherein thesecond section directly diffracts the second light beam portion of theincident light beam into a +1 ^(st)-order or a −1^(st)-order beam andthe second photodetector receives the diffracted second light beam. 32.The optical pickup as recited in claim 29, wherein the third sectiondiffracts the third light beam portion of the incident light beam into a+1^(st)-order or a −1^(st)-order beam and the third photodetectorreceives the diffracted third light beam.
 33. The optical pickup asrecited in claim 29, wherein the light beam splitter comprises in thefirst section a through hole, a direct transmit portion, or a hologrampattern to diffract the incident light beam and to transmit a firstresulting order light beam to the first photodetector and comprises, inthe second and third sections, a hologram pattern to diffract theincident light beam and to transmit a second and third resulting orderlight beam to the second and third photodetector, respectively.
 34. Theoptical pickup as recited in claim 22, further comprising: a sphericalaberration compensation element on the optical path between the opticalpath changer and the objective lens, driven according to the thicknessvariation signal from the thickness variation detection circuit tocompensate for spherical aberration caused by the thickness variation ofthe recording medium.
 35. The optical pickup as recited in claim 22,further comprising: a collimating lens on the optical path between thelight source and the optical path changer, collimating a diverging lightbeam from the light source; and an actuator actuating the collimatinglens according to the thickness variation signal detected by thethickness variation detection circuit compensating for sphericalaberration caused by the thickness variation of the recording medium.36. The optical pickup as recited in claim 22, further comprising: ablue-light semiconductor laser emitting the light beam having awavelength between 400 nm and 420 nm, wherein the objective lenscomprises a numerical aperture of at least 0.7.
 37. An optical pickupfor a recording medium, comprising: a light beam division and detectionunit comprising receiving portions dividing an incident light beamreflected from the recording medium into a first light beam portion anda second light beam portion around the first light beam portion andconverting the first and second light beam portions into first andsecond detection signals, respectively; and a thickness variationdetection circuit detecting a variation in thickness of the recordingmedium according to the first and second detection signals andoutputting a thickness variation signal indicative thereof.
 38. Theoptical pickup according to claim 37, wherein the thickness variationdetection circuit detects the variation in thickness of the recordingmedium by subtracting the second detection signal from the firstdetection signal and outputs a thickness variation signal indicativethereof.
 39. The optical pickup according to claim 37, furthercomprising: a light source generating and emitting a light beam; anobjective lens focusing the light beam from the light source to form alight spot incident on the recording medium; and an optical path changerdisposed on an optical path between the light source and the objectivelens, altering a traveling path of the incident light beam.
 40. Theoptical pickup as recited in claim 37, further comprising: a sphericalaberration compensation element on the optical path between the opticalpath changer and the objective lens, driven according to the thicknessvariation signal from the thickness variation detection circuit tocompensate for spherical aberration caused by the thickness variation ofthe recording medium.
 41. The optical pickup as recited in claim 37,further comprising: a collimating lens on the optical path between thelight source and the optical path changer, collimating a diverging lightbeam from the light source; and an actuator actuating the collimatinglens according to a thickness variation signal detected by the thicknessvariation detection circuit compensating for the spherical aberrationcaused by the thickness variation of the recording medium.
 42. Theoptical pickup as recited in claim 39, wherein the light beam divisionand detection unit further divides the incident light into a third lightbeam portion around the first light beam portion and converts the thirdlight beam portions into a third detection signal and the thicknessvariation detection circuit detects the variation in thickness of therecording medium by a sum of the second and third detection signals fromthe first detection signal and outputting the thickness variation signalindicative thereof.
 43. The optical pickup as recited in claim 39,wherein the thickness variation detection circuit amplifies at least oneof the first and second detection signals by a predetermined gainfactor, and processes the at least one of the first and second detectionsignals to detect the thickness variation of the recording medium. 44.The optical pickup as recited in claim 39, wherein the thickness of therecording medium is a distance between a light incident surface of therecording medium and an information recording surface of the recordingmedium.
 45. The optical pickup as recited in claim 39, wherein theoptical path changer comprises: a beam splitter transmitting the lightbeam from the light source to the recording medium through the objectivelens and reflecting the incident light beam through the objective lensto the photodetector by a predetermined ratio.
 46. The optical pickup asrecited in claim 39, further comprising: a collimating lens on theoptical path between the light source and the optical path changercollimating a diverging light beam from the light source; and a sensinglens on the optical path between the optical path changer and the lightbeam division and detection unit condensing the incident light beam. 47.The optical pickup as recited in claim 39, wherein the optical pathchanger comprises: a polarization beam splitter selectively transmittingthe light beam from the light source to the recording medium andreflecting the incident light beam to the photodetector according to apolarization of the incident light beam; and a quarter-wave platedisposed on the optical path between the polarization beam splitter andthe objective lens changing a phase of the incident light beam.
 48. Theoptical pickup as recited in claim 39, wherein the thickness variationdetection circuit further comprises: a gain controller amplifying atleast one of the first and second detection signals by a predeterminedgain factor k prior to subtracting the second detection signal from thefirst detection signal, adjusting an offset of the thickness variationsignal.
 49. The optical pickup as recited in claim 39, wherein the lightbeam division and detection unit comprises: a light beam splittercomprising a first section and a second section dividing the incidentlight beam into the first light beam portion and the second light beamportion around the first light beam portion; and a photodetectorreceiving and photoelectrically converting the first and second lightbeam portions into the first and second detection signals, respectively.50. The optical pickup as recited in claim 39, wherein the first sectiontransmits the first light beam portion of the incident light beam ordiffracts the first light beam into a 0^(th)-order beam and thephotodetector receives the transmitted or diffracted first light beam.51. The optical pickup as recited in claim 39, wherein the secondsection diffracts the second light beam portion of the incident lightbeam into a +1^(st)-order or −1^(st)-order beam and the photodetectorreceives the diffracted second light beam.
 52. The optical pickup asrecited in claim 39, wherein the light beam splitter comprises in thefirst section a through hole, a direct transmit portion, or a hologrampattern to diffract the incident light beam and to transmit a resulting0^(th)-order light beam to the photodetector, and comprises in thesecond section a hologram pattern to diffract the incident light beamand to transmit a resulting +1^(st)-order or −1^(st)-order light beam tothe photodetector.
 53. The optical pickup as recited in claim 39,further comprising: a blue-light semiconductor laser emitting the lightbeam having a wavelength between 400 nm and 420 nm, wherein theobjective lens comprises a numerical aperture of at least 0.7.
 54. Anoptical pickup for a recording medium, comprising: a light sourcegenerating and emitting a light beam; an objective lens focusing thelight beam from the light source to form a light spot on the recordingmedium; an optical path changer disposed on an optical path between thelight source and the objective lens, altering a traveling path of thelight beam incident on the recording medium; a light beam division anddetection unit dividing the incident light beam passed through theobjective lens and the optical path changer after being reflected fromthe recording medium into a first light beam portion and a second lightbeam portion around the first light beam portion, and detecting firstand second detection signals from the first and second light beamportions; and a thickness variation detection circuit detecting avariation in thickness of the recording medium by subtracting the seconddetection signal from the first detection signal and outputting athickness variation signal indicative thereof to compensate forspherical aberration, wherein the optical pickup is exclusive of anastigmatism lens causing astigmatism affecting the light beam passedback through the objective lens, and the optical path changer afterbeing reflected from the recording surface of the optical disc.